1. Field of the Invention
The present invention relates to the Duffy receptor of a Plasmodium, particularly Plasmodium knowlesi and Plasmodium vivax Duffy receptors.
2. Background Information
Duffy positive blood group determinants on human erythrocytes are essential for invasion of human erythrocytes by plasmodium vivax, a human malaria, and by P. knowlesi, a malaria of old world monkeys that invades human erythrocytes [Miller et al., Science 189:561-563 (1975) and Miller et al., N. Engl. J. Med. 295:302-304 (1976)]. During invasion, a merozoite first attaches to an erythrocyte on any surface of the merozoite, then reorients such that its apical end is in contact with the erythrocyte [Dvorak et al., Science 187:748-750 (1975)]. Both attachment and reorientation of P. knowlesi merozoites occur equally well on Duffy positive and negative erythrocytes [Miller et al., Science 189:561-563 (1975)]. A junction then forms between the apical end of the merozoite and the Duffy-positive erythrocyte, followed by vacuole formation and entry of the merozoite into the vacuole [Aikawa et al., J. Cell Biol. 77:72-82 (1978)]. Junction formation and merozoite entry into the erythrocyte do not occur on Duffy negative cells [Miller et al., J. Exp. Med. 149:172-184 (1979)], suggesting that a receptor specific for the Duffy determinant is involved in apical junction formation but not initial attachment.
The apical end of merozoites is defined by the presence of three organelles: rhoptries, dense granules, and micronemes. The contents of the rhoptries discharge onto the erythrocyte membrane after apical junction formation, presumably to form the vacuole into which the parasite enters [Ladda et al., (1969); Bannister et al., (1975) and Aikawa et al., (1978)]. The dense granules release substances into the vacuolar space after the apical end has entered the vacuole [Bannister et al., (1975) and Torii et al., Infection and Immunity 57:3230-3233 (1989)]. The function of the micronemes has been unknown. Nevertheless, the location of these organelles suggest they are involved in the mechanism of invasion.
Duffy binding proteins are defined biologically as those soluble proteins that appear in the culture supernatant after the infected erythrocytes release merozoites, the invasive stage, and that bind to human Duffy positive and not to human Duffy negative erythrocytes. A 135 kDa protein in P. vivax and P. knowlesi has been studied in greatest detail. Its binding to Duffy positive erythrocytes is blocked by antisera to the Duffy blood group determinants and purified Duffy blood group antigens and it binds specifically in the region of the Duffy blood group determinant on Western blots.
A soluble 135 kDa protein from P. knowlesi culture supernatant was identified that specifically binds the Duffy blood group determinants [Haynes et al., J. Exp. Med. 167: 1873-1881 (1988) and Miller et al., Mol. Biochem. Parasitol. 31: 217-222 (1988)]. An analogous protein of 135 kDa has been found in the culture supernatant of P. vivax which binds specifically to the Duffy blood group determinants [Wertheimer et al., Exp. Parasitol. 69, 340-350 (1989) and Barnwell et al., J. Exp. Med. 169:1795-1802 (1989)]. The specificity of binding and immunochemical data indicate that the soluble 135 kDa protein or a membrane bound form of this protein is the Duffy receptor. There are four major Duffy phenotypes of human erythrocytes, Fy(a), Fy(b), Fy(ab), and Fy(negative), as defined by the anti-Fy.sup.a and anti-Fy.sup.b sera [Hadley et al., In Red Cell Antigens and Antibodies, G. Garratty, ed. (Arlington, Va.: American Association of Blood Banks) pp. 17-33 (1986)]. More P. knowlesi Duffy binding protein binds to human Duffy b erythrocytes than to Duffy a erythrocytes; none bind to Duffy negative erythrocytes [Haynes et al., J. Exp. Med. 167:1873-1881 (1988)]. The P. vivax Duffy binding proteins bind equally to Fya and Fyb erythrocytes. The P. knowlesi Duffy binding proteins also bind to rhesus erythrocytes, but the P. vivax Duffy binding proteins do not bind to rhesus erythrocytes. This binding correlates with the susceptibility of these erythrocytes to invasion by P. knowlesi and P. vivax. The binding specificities probably reflect the differences in the Duffy blood group antigens of the host to which the parasite has adapted.